Splitting between Bright and Dark excitons in Transition Metal Dichalcogenide Monolayers

TL;DR

This paper investigates the energy splitting between bright and dark excitons in transition metal dichalcogenide monolayers using advanced computational methods, highlighting the role of spin-orbit coupling and Coulomb exchange effects.

Contribution

It provides a detailed first-principles calculation of bright-dark exciton splitting, emphasizing the impact of intra-valley Coulomb exchange and spin-orbit coupling.

Findings

Bright-dark exciton splitting is significantly influenced by intra-valley Coulomb exchange.

Spin-orbit coupling affects the optical spectra and exciton fine structure.

The calculations clarify the mechanisms behind exciton emission properties in TMD monolayers.

Abstract

The optical properties of transition metal dichalcogenide monolayers such as the two-dimensional semiconductors MoS$_2$ and WSe$_2$ are dominated by excitons, Coulomb bound electron-hole pairs. The light emission yield depends on whether the electron-hole transitions are optically allowed (bright) or forbidden (dark). By solving the Bethe Salpeter Equation on top of $GW$ wave functions in density functional theory calculations, we determine the sign and amplitude of the splitting between bright and dark exciton states. We evaluate the influence of the spin-orbit coupling on the optical spectra and clearly demonstrate the strong impact of the intra-valley Coulomb exchange term on the dark-bright exciton fine structure splitting.